On the origin of increase in substrate current and impact ionization efficiency in strained-Si n- and p-MOSFETs

Strain dependence of substrate current and impact ionization efficiency in the strained-Si n- and p-MOSFETs has been systematically investigated with the wide range of strain, by utilizing the universal relationship between the impact ionization efficiency and the electrical field in the pinch-off region. It was found that the impact ionization efficiency increased with increasing strain, leading to the drastic increase of substrate current, and that the strain dependence of impact ionization efficiency was well represented by simply using the strain dependence of the bandgap energy regardless of carrier type. This result strongly suggests that the strain-induced enhancement of impact ionization efficiency is not due to the modification of the carrier transport properties but to the narrowing of the bandgap, and presumably, the resulting decrease in threshold energy for impact ionization. It was also found that the impact ionization efficiency increased with the increase of temperature while its dependence did not exhibit distinct strain dependence. These results are attributable mainly to the temperature dependence of energy distribution function.